his contemporaries. His mission towards antiquity reminds us of Scott, but he is, as a poet, a better artist than Scott; he has sometimes touches of exquisite diction and of overwrought sensibility which recall Coleridge to us. In his wide ambition and profuseness he possessed some characteristics of Southey, although his style has far more vitality. With all his faults he was a very great writer, and one of the principal pioneers of the romantic movement in Europe. (E. G.)
OHLIGS, a town of Germany, in the Prussian Rhine Province,
17 m. by rail N. of Cologne, on the railway to Elberfeld. Pop.
(1905) 24,264. Its chief manufactures are cutlery and hardware,
and there are iron-foundries and flour-mills. Other industries
are brewing, dyeing, weaving and brick-making. Before 1891
it was known as Merscheid.
OHM, GEORG SIMON (1787–1854), German physicist, was born at Erlangen on the 16th of March 1787, and was educated at the university there. He became professor of mathematics in the Jesuits’ college at Cologne in 1817 and in the polytechnic school of Nuremberg in 1833, and in 1852 professor of experimental physics in the university of Munich, where he died on the 7th of July 1854. His writings were numerous, but, with one important exception, not of the first order. The exception is his pamphlet published in Berlin in 1827, with the title Die galvanische Kette mathematisch bearbeitet. This work, the germs of which had appeared during the two preceding years in the journals of Schweigger and Poggendorff, has exerted most important influence on the whole development of the theory and applications of current electricity, and Ohm’s name has been incorporated in the terminology of electrical science. Nowadays “Ohm’s Law,” as it is called, in which all that is most valuable in the pamphlet is summarized, is as universally known as anything in physics. The equation for the propagation of electricity formed on Ohm’s principles is identical with that of J. B. J. Fourier for the propagation of heat; and if, in Fourier’s solution of any problem of heat-conduction, we change the word “temperature” to “potential” and write “electric current” instead of “flux of heat,” we have the solution of a corresponding problem of electric conduction. The basis of Fourier’s work was his clear conception and definition of conductivity. But this involves an assumption, undoubtedly true for small temperature-gradients, but still an assumption, viz. that, all else being the same, the flux of heat is strictly proportional to the gradient of temperature. An exactly similar assumption is made in the statement of Ohm’s law, i.e. that, other things being alike, the strength of the current is at each point proportional to the gradient of potential. It happens, however, that with our modern methods it is much more easy to test the accuracy of the assumption in the case of electricity than in that of heat; and it has accordingly been shown by J. Clerk Maxwell and George Chrystal that Ohm’s law is true, within the limits of experimental error, even when the currents are so powerful as almost to fuse the conducting wire.
OHMMETER, an electrical instrument employed for measuring insulation-resistance or other high electrical resistances. For the purpose of measuring resistances up to a few thousand ohms, the most convenient appliance is a Wheatstone’s Bridge (q.v.), but when the resistance of the conductor to be measured is several hundred thousand ohms, or if it is the resistance of a so-called insulator, such as the insulating covering of the copper wires employed for distributing electric current in houses and buildings for electric lighting, then the ohmmeter is more convenient. An ohmmeter in one form consists of two pairs of coils, one pair called the series coil and the other called the shunt coil. These coils are placed with their axes at right angles to one another, and at the point where the axes intersect a small pivoted needle of soft iron is placed, carrying a longer index needle moving over a scale.
Suppose it is desired to measure the insulation-resistance of a
Fig. 1.
system of electric house wiring; the ohmmeter circuits are then joined
up as shown in fig. 1, where W represents a portion of the wiring
of the building and I a portion of the insulating materials surrounding
it. The object of the test is to discover the resistance of the insulator
I, that is, to determine how much current flows through this insulator
by leakage under a certain electromotive force or voltage which must
not be less than that which will be employed in practice when the
electric lights supplied through these wires are in operation.
For this purpose the ohmmeter is provided with a small dynamo D,
contained in a box, which produces a continuous electromotive
force of from 200 to 500 volts
when the handle
of the instrument is
steadily turned. In
making the test, the
whole of the copper
wires belonging to any
section of the wiring and
the test must be connected
together at some
point and then connected
through the series
coil of the ohmmeter
with one terminal of the
dynamo. The shunt coil
Sh and the series coil Se
are connected together
at one point, and the
remaining terminals of
the dynamo and shunt coil must be connected to a “good
earth,” which is generally the gas or water pipes w of the
building. On setting the dynamo in operation, a current passes
through the shunt coil of the ohmmeter proportional to the voltage
of the dynamo, and, if there is any sensible leakage through the
insulator to earth, at the same time another current passes through
the series coil proportional to the conductivity of the insulation of
the wiring under the electromotive force used. The two coils, the
shunt and the series coil, then produce two magnetic fields, with
their lines of force at right angles to one another. The small pivoted
iron needle ns placed in their common field therefore takes up a
certain position, dependent on the relative value of these fields.
The tangent of the angle of deflection θ of this needle measured from
its position, when the shunt coil is disconnected, is equal to the ratio
of the voltage of the dynamo to the current through the insulator. If
we call this last resistance R, the voltage of the working dynamo V,
and the current through the insulator C, then tan θ＝C/V＝R.
Hence the deflection of the needle is proportional to the insulation
resistance, and the scale can be graduated to show directly this
resistance in megohms.
The Evershed and Vignoles form of the instrument is much used in testing the insulation resistance of electric wiring in houses. In this case the dynamo and ohmmeter are combined in one instrument. The field magnet of the dynamo has two gaps in it. In one the exciting armature is rotated, producing the working voltage of 250, 500 or 1000 volts. In the other gap are pivoted two coils wound on an iron core and connected at nearly a right angle to each other. One of these coils is in series with the armature circuit and with the insulation or high resistance to be measured. The other is a shunt across the terminals of the armature. When the armature is rotated, these two coils endeavour to place themselves in certain directions in the field so as to be perforated by the greatest magnetic flux. The exact position of the core, and, therefore, of an index needle connected with it, is dependent on the ratio of the voltage applied to the terminals of the high resistance or insulator and the current passing through it. This, however, is a measure of the insulation-resistance. Hence the instrument can be graduated to show this directly.
In the Nalder ohmmeter the electrostatic principle is employed. The instrument consists of a high-voltage continuous-current dynamo which creates a potential difference between the needle and the two quadrants of a quadrant electrometer (see Electrometer). These two quadrants are interconnected by the high resistance to be measured, and, therefore, themselves differ in potential. The exact position taken up by the needle is therefore determined by the potential difference (P.D.) of the quadrants and the P.D. of the needle and each quadrant, and, therefore, by the ratios of the P.D. of the ends of the insulator and the current flowing through it, that is, by its insulation resistance.
The ohmmeter recommends itself by its portability, but in default of the possession of an ohmmeter the insulation-resistance can be measured by means of an ordinary mirror galvanometer (see Galvanometer) and insulated battery of suitable voltage. In this case one terminal of the battery is connected to the earth, and the other terminal is connected through the galvanometer with the copper wire, the insulation of which it is desired to test. If any sensible current flows through this insulator the galvanometer will show a deflection.
The meaning of this deflection can be interpreted as follows: If a galvanometer has a resistance R and is shunted by a shunt of resistance S, and the shunted galvanometer is placed in series with a large resistance R′ of the order of a megohm, and if the same
